Spatially patterned polarization compensator

ABSTRACT

A modulation optical system ( 40 ) provides modulation of an incident light beam. A wire grid polarization beamsplitter ( 240 ) receives the beam of light ( 130 ) and transmits a beam of light having a first polarization, and reflects a beam of light having a second polarization orthogonal to the first polarization. Sub-wavelength wires ( 250 ) on the wire grid polarization beamsplitter face a reflective spatial light modulator. The reflective spatial light modulator receives the polarized beam of light and selectively modulates the polarized beam of light to encode data thereon. The reflective spatial light modulator reflects back both the modulated light and the unmodulated light to the wire grid polarization beamsplitter. The wire grid polarization beamsplitter separates the modulated light from the unmodulated light. A compensator ( 260 ) is located between the wire grid polarization beamsplitter and the reflective spatial light modulator ( 210 ). The compensator conditions the polarization states of the oblique and skew rays of the modulated beam and includes a spatially variant retardance that corrects for a spatially variant retardance of the reflective spatial light modulator.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of application Ser. No. 10/712,172, filed Nov. 13,2003, now U.S. Pat. No. 6,900,866, which is a continuation-in-part ofapplication Ser. No. 10/040,663, filed Jan. 7, 2002, now U.S. Pat. No.6,909,473.

FIELD OF THE INVENTION

This invention generally relates to digital projection apparatusemploying liquid crystal devices for image forming and more particularlyto an apparatus and method for achieving high levels of contrast byusing a wire grid polarization beam splitter with a compensator forminimizing leakage light in the pixel black (OFF) state.

BACKGROUND OF THE INVENTION

In order to be considered as suitable replacements for conventional filmprojectors, digital projection systems must meet demanding requirementsfor image quality. This is particularly true for cinematic projectionsystems. To provide a competitive alternative to conventionalcinematic-quality projectors, digital projection apparatus, provide highresolution, wide color gamut, high brightness (>10,000 screen lumens),and frame-sequential system contrast ratios exceeding 1,000:1.

The most promising solutions for digital cinema projection employ one oftwo types of spatial light modulators as image forming devices. Thefirst type of spatial light modulator is the digital micromirror device(DMD), developed by Texas Instruments, Inc., Dallas, Tex. DMD devicesare described in a number of patents, for example U.S. Pat. Nos.4,441,791; 5,535,047; 5,600,383 (all to Hornbeck); and U.S. Pat. No.5,719,695 (Heimbuch). Optical designs for projection apparatus employingDMDs are disclosed in U.S. Pat. No. 5,914,818 (Tejada et al.); U.S. Pat.No. 5,930,050 (Dewald); U.S. Pat. No. 6,008,951(Anderson); and U.S. Pat.No. 6,089,717 (Iwai). Although DMD-based projectors demonstrate somecapability to provide the necessary light throughput, contrast ratio,and color gamut, current resolution limitations (1024×768 pixels) andhigh component and system costs have restricted DMD acceptability forhigh-quality digital cinema projection.

The second type of spatial light modulator used for digital projectionis the liquid crystal device (LCD). The LCD forms an image as an arrayof pixels by selectively modulating the polarization state of incidentlight for each corresponding pixel. At high resolution, large area LCDscan be fabricated more readily than DMDs. LCDs are a viable alternativemodulator technology to be used in digital cinema projection systems.Among examples of electronic projection apparatus that utilize LCDspatial light modulators are those disclosed in U.S. Pat. No. 5,808,795(Shimomura et al.); U.S. Pat. No. 5,798,819 (Hattori et al.); U.S. Pat.No. 5,918,961 (Ueda); U.S. Pat. No. 6,010,121 (Maki et al.); and U.S.Pat. No. 6,062,694 (Oikawa et al.). Recently, JVC demonstrated anLCD-based projector capable of high-resolution (providing 2,000×1280pixels), high frame sequential contrast (in excess of 1000:1), and highlight throughput (nominally, up to 12,000 lumens). This system utilizedthree vertically aligned (VA) (also referred as homeotropic) LCDs (oneper color) driven or addressed by cathode ray tubes (CRTs). While thissystem demonstrated the potential for an LCD based digital cinemaprojector, system complexity and overall reliability remain concerns. Inaddition, cost considerations render such a system not yet ready forbroad commercialization in the digital cinema projection market.

JVC has also developed a new family of vertically aligned LCDs, whichare directly addressed via a silicon backplane (LCOS), rather thanindirectly by a CRT. While these new devices are promising, they havenot yet been demonstrated to fully meet the expectations for digitalcinema presentation. The JVC LCD devices are described, in part, in U.S.Pat. No. 5,652,667 (Kuragane); U.S. Pat. No. 5,767,827 (Kobayashi etal.); and U.S. Pat. No. 5,978,056 (Shintani et al.) In contrast to earlytwisted nematic or cholesteric LCDs, vertically aligned LCDs promise toprovide much higher modulation contrast ratios (in excess of 2,000:1).U.S. Pat. No. 5,620,755 (Smith et al.), also assigned to JVC,specifically describes a methodology for inducing vertical alignment inLC displays. It is instructive to note that, in order to obtain onscreen frame sequential contrast of 1,000:1 or better, the entire systemmust produce >1000:1 contrast, and both the LCDs and any necessarypolarization optics must each separately provide ˜2,000:1 contrast.Notably, while polarization compensated vertically aligned LCDs canprovide contrast >20,000:1 when modulating collimated laser beams, thesesame modulators may exhibit contrasts of 500:1 or less when modulatingcollimated laser beams without the appropriate polarizationcompensation. Modulation contrast is also dependent on the spectralbandwidth and angular width (F#) of the incident light, with contrastgenerally dropping as the bandwidth is increased or the F# is decreased.Modulation contrast within LCDs can also be reduced by residualde-polarization or mis-orienting polarization effects, such as thermallyinduces stress birefringence. Such effects can be observed in the farfield of the device, where the typically observed “iron cross”polarization contrast pattern takes on a degenerate pattern.

As is obvious to those skilled in the digital projection art, theoptical performance provided by LCD based electronic projection systemis, in large part, defined by the characteristics of the LCDs themselvesand by the polarization optics that support LCD projection. Theperformance of polarization separation optics, such as polarizationbeamsplitters, pre-polarizers, and polarizer/analyzer components is ofparticular importance for obtaining high contrast ratios. The precisemanner in which these polarization optical components are combinedwithin a modulation optical system of a projection display can also havesignificant impact on the final resultant contrast.

The most common conventional polarization beamsplitter solution, whichis used in many projection systems, is the traditional MacNeille prism,disclosed in U.S. Pat. No. 2,403,731. This device has been shown toprovide a good extinction ratio (on the order of 300:1). However, thisstandard prism operates well only with incident light over a limitedrange of angles (a few degrees). Because the MacNeille prism designprovides good extinction ratio for one polarization state only, a designusing this device must effectively discard half of the incoming lightwhen this light is from an unpolarized white light source, such as froma xenon or metal halide arc lamp.

Conventional glass polarization beamsplitter design, based on theMacNeille design, has other limitations beyond the limited angularresponse, which could impede its use for digital cinema projection. Inparticular, bonding techniques used in fabrication or thermal stress inoperation, can cause stress birefringence, in turn degrading thepolarization contrast performance of the beamsplitter. These effects,which are often unacceptable for mid range electronic projectionapplications, are not tolerable for cinema projection applications. Thethermal stress problem has recently been improved upon, with the use ofa more suitable low photo-elasticity optical glass, disclosed in U.S.Pat. No. 5,969,861 (Ueda et al.), which was specially designed for usein polarization components. Unfortunately, high fabrication costs anduncertain availability limit the utility of this solution. Furthermore,while it would be feasible to custom melt low-stress glass prisms suitedto each wavelength band in order to minimize stress birefringence, whilesomewhat expanding angular performance, such a solution is costly anderror-prone. As a result of these problems, the conventional MacNeillebased glass beamsplitter design, which is capable of the necessaryperformance for low to mid-range electronic projection systems,operating at 500–5,000 lumens with approximately 800:1 contrast, likelyfalls short of the more demanding requirements of full-scale commercialdigital cinema projection.

Other polarization beamsplitter technologies have been proposed to meetthe needs of an LCD based digital cinema projection system. For example,the beamsplitter disclosed in U.S. Pat. No. 5,912,762 (Li et al.), whichcomprises a plurality of thin film layers sandwiched between two doveprisms, attempts to achieve high extinction ratios for both polarizationstates. Theoretically, this beamsplitter device is capable of extinctionratios in excess of 2,000:1. Moreover, when designed into a projectionsystem with six LCDs (two per color), this prism could boost systemlight efficiency by allowing use of both polarizations. However, sizeconstraints and extremely tight coating tolerances present significantobstacles to commercialization of a projection apparatus using thisbeamsplitter design.

As another conventional solution, some projector designs have employedliquid-immersion polarization beamsplitters. Liquid-filled beamsplitters(see U.S. Pat. No. 5,844,722 (Stephens), for example) have been shown toprovide high extinction ratios needed for high-contrast applications andhave some advantages under high-intensity light conditions. However,these devices are costly to manufacture, must be fabricated without dustor contained bubbles and, under conditions of steady use, have exhibiteda number of inherent disadvantages. Among the disadvantages ofliquid-immersion polarization beamsplitters are variations in refractiveindex of the liquid due to temperature, including uneven indexdistribution due to convection. Leakage risk presents another potentialdisadvantage for these devices.

Wire grid polarizers have been in existence for many years, and wereprimarily used in radio-frequency and far infrared optical applications.Use of wire grid polarizers with visible spectrum light has beenlimited, largely due to constraints of device performance ormanufacture. For example, U.S. Pat. No. 5,383,053 (Hegg et al.)discloses use of a wire grid beamsplitter in a virtual image displayapparatus. In the Hegg et al. disclosure, an inexpensive wire gridbeamsplitter provides high light throughput efficiency when comparedagainst conventional prism beamsplitters. The polarization contrastprovided by the wire grid polarizer of Hegg et al. is very low (6.3:1)and unsuitable for digital projection. A second wire grid polarizer forthe visible spectrum is disclosed in U.S. Pat. No. 5,748,368 (Tamada).While the device discussed in this patent provides polarizationseparation, the contrast ratio is inadequate for cinematic projectionand the design is inherently limited to rather narrow wavelength bands.

Recently, as is disclosed in U.S. Pat. No. 6,122,103 (Perkins et al.);U.S. Pat. No. 6,243,199 (Hansen et al.); and U.S. Pat. No. 6,288,840(Perkins et al.), high quality wire grid polarizers and beamsplittershave been developed for broadband use in the visible spectrum. These newdevices are commercially available from Moxtek Inc. of Orem, Utah. Whileexisting wire grid polarizers, including the devices described in U.S.Pat. Nos. 6,122,103 and 6,243,199 may not exhibit all of the necessaryperformance characteristics needed for obtaining the high contrastrequired for digital cinema projection, these devices do have a numberof advantages. When compared against standard polarizers, wire gridpolarization devices exhibit relatively high extinction ratios and highefficiency. Additionally, the contrast performance of these wire griddevices also has broader angular acceptance (NA or numerical aperture)and more robust thermal performance with less opportunity for thermallyinduced stress birefringence than standard polarization devices.Furthermore, the wire grid polarizers are robust relative to harshenvironmental conditions, such as light intensity, temperature, andvibration. These devices perform well under conditions of differentcolor channels, with the exception that response within the blue lightchannel may require additional compensation.

Wire grid polarization beamsplitter (PBS) devices have been employedwithin some digital projection apparatus. For example, U.S. Pat. No.6,243,199 (Hansen et al.) discloses use of a broadband wire gridpolarizing beamsplitter for projection display applications. U.S. Pat.No. 6,234,634 (also to Hansen et al.) discloses a wire grid polarizingbeamsplitter that functions as both polarizer and analyzer in a digitalimage projection system. U.S. Pat. No. 6,234,634 states that very loweffective F#'s can be achieved using wire grid PBS, with some loss ofcontrast, however. Notably, U.S. Pat. No. 6,234,634 does not discuss howpolarization compensation may be used in combination with wire griddevices to reduce light leakage and boost contrast for fast opticalsystems operating at low F#'s.

In general, wire grid polarizers have not yet been satisfactorily provento meet all of the demanding requirements imposed by digital cinemaprojection apparatus, although progress is being made. Deficiencies insubstrate flatness, in overall polarization performance, and inrobustness at both room ambient and high load conditions have limitedcommercialization of wire grid polarization devices for cinematicprojection.

Of particular interest and relevance for the apparatus and methods ofthe present invention, it must be emphasized that individually neitherthe wire grid polarizer, nor the wire grid polarization beamsplitter,provide the target polarization extinction ratio performance(nominally >2,000:1) needed to achieve the desired projection systemframe sequential contrast of 1,000:1 or better, particularly at smallF#'s (<F/3.5). Rather, both of these components provide less than˜1,200:1 contrast under the best conditions. Significantly, performancefalls off further in the blue spectrum. Therefore, to achieve thedesired 2,000:1 contrast target for the optical portion of the system(excluding the LCDs), it is necessary to utilize a variety ofpolarization devices, including possibly wire grid polarization devices,in combination within a modulation optical system of the projectiondisplay. However, the issues of designing an optimized configuration ofpolarization optics, including wire grid polarizers, in combination withthe LCDs, color optics, and projection lens, have not been completelyaddressed either for electronic projection in general, or for digitalcinema projection in particular. Moreover, the prior art does notdescribe how to design a modulation optical system for a projectiondisplay using both LCDs and wire grid devices, which further haspolarization compensators to boost contrast.

There are numerous examples of polarization compensators developed toenhance the polarization performance of LCDs generally, and verticallyaligned LCDs particularly. In an optimized system, the compensators aresimultaneously designed to enhance the performance of the LCDs and thepolarization optics in combination. These compensators typically provideangular varying birefringence, structured in a spatially variantfashion, to affect polarization states in portions (within certainspatial and angular areas) of the transiting light beam, withoutaffecting the polarization states in other portions of the light beam.Polarization compensators have been designed to work with LCDsgenerally, but also vertically aligned LCDs in particular. U.S. Pat. No.4,701,028 (Clerc et al.) discloses birefringence compensation designedfor a vertically aligned LCD with restricted thickness. U.S. Pat. No.5,039,185 (Uchida et al.) discloses a vertically aligned LCD withcompensator comprising at least two uniaxial or two biaxial retardersprovided between a sheet polarizer/analyzer pair. U.S. Pat. No.5,298,199 (Hirose et al.) discloses the use of a biaxial filmcompensator correcting for optical birefringence errors in the LCD, usedin a package with crossed sheet polarizers, where the LCD dark state hasa non-zero voltage (a bias voltage). U.S. Pat. No. 6,081,312 (Aminaka etal.) discloses a discotic film compensator which is designed to optimizecontrast for a voltage ON state of the VA LCD. By comparison, U.S. Pat.No. 6,141,075 (Ohmuro et al.) discloses a VA LCD compensated by tworetardation films, one with positive birefringence and the other withnegative birefringence.

U.S. Pat. No. 5,576,854 (Schmidt et al.) discloses a compensatorconstructed for use in projector apparatus using an LCD with theconventional MacNeille prism type polarization beamsplitter. Thiscompensator comprises a ¼ wave plate for compensating the prism and anadditional 0.02 λ's compensation for the inherent LCD residualbirefringence effects. U.S. Pat. No. 5,619,352 (Koch et al.) disclosescompensation devices, usable with twisted nematic LCDs, where thecompensators have a multi-layer construction, using combinations ofA-plates, C-plates, and O-plates, as needed.

In general, most of these prior art compensator patents assume the LCDsare used in combination with sheet polarizers, and correct for the LCDpolarization errors. However, polarization compensators have also beenexplicitly developed to correct for non-uniform polarization effectsfrom the conventional Polaroid type dye sheet polarizer. The dye sheetpolarizer, developed by E. H. Land in 1929 functions by dichroism, orthe polarization selective anistropic absorption of light. Compensatorsfor dye sheet polarizers are described in Chen et al. (J. Chen, K. -H.Kim, J. -J. Kyu, J. H. Souk, J. R. Kelly, P. J. Bos, “Optimum FilmCompensation Modes for TN and VA LCDs”, SID 98 Digest, pgs. 315–318.),and use a combination A-plate and C-plate construction. The maximumcontrast of the LCD system aimed at in prior art patents such as in U.S.Pat. No. 6,141,075 (Ohmuro et al.) is only up to 500:1, which issufficient for many applications, but does not meet the requirement ofdigital cinema projection.

While this prior art material extensively details the design ofpolarization compensators used under various conditions, compensatorsexplicitly developed and optimized for use with wire grid polarizers arenot disclosed. Furthermore, the design of polarization compensators toenhance the performance of a modulation optical system using multiplewire grid polarizer devices, or using multiple wire grid devices incombination with vertically aligned LCDs, have not been previouslydisclosed. Without compensation, the wire grid polarization beamsplitterprovides acceptable contrast when incident light is within a lownumerical aperture. However, in order to achieve high brightness levels,it is most advantageous for an optical system to have a high numericalaperture (>˜0.13), so that it is able to gather incident light at largeroblique angles. The conflicting goals of maintaining high brightness andhigh contrast ratio present a significant design problem forpolarization components. Light leakage in the OFF state must be minimalin order to achieve high contrast levels. Yet, light leakage is mostpronounced for incident light at the oblique angles required forachieving high brightness.

Compensator requirements for wire grid polarizing beamsplitter devicesdiffer significantly from more conventional use of compensators withpolarizing beamsplitter devices based on the MacNeille prism design aswas noted in reference to U.S. Pat. No. 5,576,854. Performance resultsindicate that the conventional use of a ¼ wave plate compensator is nota solution and can even degrade contrast ratio. Additionally, whilecompensators have previously been specifically developed to work intandem with VA LCDs in projection display systems, compensatorsoptimized for use with VA LCDs in the context of a modulation opticalsystem which utilizes wire grid polarization beamsplitters have not beendeveloped and disclosed.

Thus it can be seen that there is a need for an improved projectionapparatus that uses wire grid polarization devices, vertically alignedLCDs, and polarization compensators in combination to providehigh-contrast output.

SUMMARY OF THE INVENTION

Briefly, according to one aspect of the present invention a modulationoptical system for providing modulation of an incident light beamcomprises a wire grid polarization beamsplitter for receiving the beamof light, for transmitting light from the beam of light having a firstpolarization, and for reflecting light of the beam of light having asecond polarization orthogonal to the first polarization, whereinsub-wavelength wires on the wire grid polarization beamsplitter face areflective spatial light modulator. The reflective spatial lightmodulator receives the polarized beam of light, having either a firstpolarization or a second polarization, and selectively modulates thepolarized beam of light to encode data thereon, providing both modulatedlight and unmodulated light which differ in polarization. The reflectivespatial light modulator reflects back both the modulated light and theunmodulated light to the wire grid polarization beamsplitter. The wiregrid polarization beamsplitter then separates the modulated light fromthe unmodulated light. A compensator is located between the wire gridpolarization beamsplitter and the reflective spatial light modulator,wherein the compensator is provided for conditioning the polarizationstates of the oblique and skew rays of the modulated beam. Thecompensator includes a spatially variant retardance that corrects for aspatially variant retardance of the reflective spatial light modulator.

The invention and its objects and advantages will become more apparentin the detailed description of the preferred embodiment presented below.

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming the subject matter of the present invention, itis believed that the invention will be better understood from thefollowing description when taken in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is a schematic view showing an arrangement of optical componentsin a projection apparatus;

FIG. 2 is a perspective view of a prior art wire grid polarizer.

FIG. 3 is a cross sectional view showing a modulation optical systemwhich includes a wire grid polarization beamsplitter.

FIG. 4 is a graph showing the relationship of contrast ratio to F/# fora modulation optical system which includes both a wire grid polarizationbeamsplitter and an LCD, both with and without polarizationcompensation.

FIG. 5 a shows the geometry of incident light relative to the wire gridpolarizing beamsplitter and an LCD within a modulation optical system,illustrating both polarization states and the local beam geometry.

FIG. 5 b illustrates the geometry of normally incident light relative tothe polarization states of crossed polarizers.

FIG. 5 c illustrates the geometry of an unfolded modulation opticalsystem with a transmissive spatial light modulator, wire gridpolarizers, and a polarization compensator.

FIGS. 6 a and 6 b show the angular response for crossed wire gridpolarizers without polarization compensation.

FIGS. 7 a–e show the possible axial orientations and construction of apolarization compensator.

FIGS. 8 a–i are the far field angular response plots from variousarrangements of wire grid polarization devices and compensators.

FIG. 9 a shows the contrast contour plot for an ideal VA LCD withoutcompensator.

FIG. 9 b shows the contrast contour plot for a VA LCD with 10 nm inducedretardation from ITO substrate.

FIG. 9 c shows the contrast contour plot for a VA LCD with 10 nm inducedretardation from ITO substrate and with proper compensator.

FIG. 10 is a schematic view showing the basic components of a modulationoptical system according to the preferred embodiment of the presentinvention.

FIG. 11 shows a contrast pattern with a high contrast ratio in thecenter.

FIG. 12 shows a schematic side view of a patterned compensator accordingto the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present description is directed in particular to elements formingpart of, or cooperating more directly with, apparatus in accordance withthe invention. It is to be understood that elements not specificallyshown or described may take various forms well known to those skilled inthe art.

Referring to FIG. 1, there is shown in schematic form the arrangement ofoptical components in a digital projection apparatus 10, as described incommonly-assigned U.S. Pat. No. 6,585,378 (Kurtz et al.), the disclosureof which is incorporated herein. Illumination optics 20 andpre-polarizer 45 precondition light from a light source 15 to provideillumination that is essentially uniformized and polarized. Illuminationoptics 20 includes uniformizing optics, such as an integrating bar or afly's eye integrator assembly, and condensing relay optics assembly.This light is subsequently polarized by pre-polarizer 45, with light ofthe desired polarization state directed towards the polarizationbeamsplitter, while the rejected alternate polarization state lightnominally reflects back towards the light source. Pre-polarizer 45 ispart of modulation optical system 40, which also comprises a wire gridpolarization beamsplitter 50, a polarization rotating spatial lightmodulator 55, and a polarization analyzer 60. Nominally, wire gridpolarization beamsplitter 50 transmits the incident light having thepreferred polarization state, while reflecting residual incident lighthaving the alternate polarization state out of the system. Incidentlight is modulated by spatial light modulator 55, which is nominally aliquid crystal display (LCD), to encode a two-dimensional image onto thelight, which is then reflected as a modulated light beam. Wire gridpolarization beamsplitter 50 reflects light from the modulated lightbeam having one polarization state, and transmits the light having thealternate polarization state. Projection optics 70 then directs thereflected modulated light beam onto a display surface 75, which isnominally a projection screen.

The design of digital projection apparatus 10 and modulation opticalsystem 40 both can be better understood from a deeper discussion of theproperties of the wire grid polarizers used within these systems. FIG. 2illustrates a basic prior art wire grid polarizer and defines terms thatwill be used in a series of illustrative examples of the prior art andthe present invention. The wire grid polarizer 100 is comprised of amultiplicity of parallel conductive electrodes (wires) 110 supported bya dielectric substrate 120. This device is characterized by the gratingspacing or pitch or period of the conductors, designated (p); the widthof the individual conductors, designated (w); and the thickness of theconductors, designated (t). Nominally, a wire grid polarizer usessub-wavelength structures, such that the pitch (p), conductor or wirewidth (w), and the conductor or wire thickness (t) are all less than thewavelength of incident light (λ). A beam of light 130 produced by alight source 132 is incident on the polarizer at an angle θ from normal,with the plane of incidence orthogonal to the conductive elements. Thewire grid polarizer 100 divides this beam into specular non-diffractedoutgoing light beams; reflected light beam 140 and transmitted lightbeam 150. The definitions for S and P polarization used are that Spolarized light is light with its polarization vector parallel to theconductive elements, while P polarized light has its polarization vectororthogonal to the conductive elements. In general, a wire grid polarizerwill reflect light with its electric field vector parallel (“S”polarization) to the grid, and transmit light with its electric fieldvector perpendicular (“P” polarization) to the grid. Wire grid polarizer100 is a somewhat unusual polarization device, in that it is an E-typepolarizer in transmission (transmits the extraordinary ray) and O-typepolarizer in reflection (reflects the ordinary ray).

When such a device is used at normal incidence (θ=0 degrees), thereflected light beam 140 is generally redirected towards the lightsource 132, and the device is referred to as a polarizer. However, whensuch a device is used at non-normal incidence (typically 30°<θ<60°), theilluminating beam of light 130, the reflected light beam 140, and thetransmitted light beam 150 follow distinct separable paths, and thedevice is referred to as a polarization beamsplitter. The detaileddesign of a wire grid device, relative to wire pitch (p), wire width(w), wire duty cycle (w/p), and wire thickness (t), may be optimizeddifferently for use as a polarizer or a polarization beamsplitter. Itshould be understood that both digital projection apparatus 10 andmodulation optical system 40, when designed with polarization modifyingspatial light modulators, may use polarization analyzers andpolarization beamsplitters other than wire grid type devices. Forexample, the polarization beamsplitter may be a MacNeille type glassprism, or the polarizer may be either a dye/polymer based sheetpolarizer. Within this discussion, the polarizing beamsplitter isassumed to be a wire grid type device, while both the pre-polarizer 45and analyzer 60 are also generally considered to be wire grid devices aswell, although that is not required for all configurations for theprojector.

The preferred spatial relationships of these polarizers, as used in amodulation optical system 200, are illustrated in FIG. 3. The basicstructure and operation of modulation optical system 200 are describedin commonly-assigned U.S. Pat. No. 6,585,378, the disclosure of which isincorporated herein. Modulation optical system 200, which is a portionof an electronic projection system, comprises an incoming illuminationlight beam 220, focused through pre-polarizer 230, wire gridpolarization beamsplitter 240, a compensator 260, and onto spatial lightmodulator 210 (the LCD) by a condenser 225. A modulated, image-bearinglight beam 290 is reflected from the surface of spatial light modulator210, transmitted through compensator 260, reflected off the near surfaceof wire grid polarization beamsplitter 240, and is subsequentlytransmitted through a polarization analyzer 270. After leavingmodulation optical system 200, modulation image bearing light beam 290follows along optical axis 275, and is transmitted through recombinationprism 280 and projection lens 285 on its way to the screen.Pre-polarizer 230 and polarization analyzer are assumed to both be wiregrid polarization devices. A full color projection system would employone modulation optical system 200 per color (red, green, and blue), withthe color beams re-assembled through the recombination prism 280.Condensor 225, which will likely comprise several lens elements, is partof a more extensive illumination system which transforms the sourcelight into a rectangularly shaped region of nominally uniform lightwhich nominally fills the active area of spatial light modulator 210.

In a modulation optical system 200 utilizing a prior art wire gridpolarization beamsplitter, the wire grid polarization beamsplitter 240consists of a dielectric substrate 245 with sub-wavelength wires 250located on one surface (the scale of the wires is greatly exaggerated).Wire grid polarization beamsplitter 240 is disposed for reflection intoprojection lens system 285, thereby avoiding the astigmatism and comaaberrations induced by transmission through a tilted plate. Compensator260 is nominally a waveplate which provides a small amount of retardanceneeded to compensate for geometrical imperfections and birefringenceeffects which originate at the surface of spatial light modulator 210.For example, as discussed in U.S. Pat. No. 5,576,854 (Schmidt et al),compensator 260 may provide 0.02 λ's of retardance (A-plate) to correctfor polarization errors caused by residual geometrical imperfections ofthe LCD polarizing layer and residual thermally induced birefringencewithin the counter electrode substrate within the LCD package. In lessdemanding applications than digital cinema, compensator 260 may proveoptional.

The construction of modulation optical system 200, as used in a digitalcinema application, is defined both by the system specifications and thelimitations of the available wire grid polarizing devices. Inparticular, digital cinema requires the electronic projector to providehigh frame sequential system contrast (1,000:1 or better). To accomplishthis, the polarization optical components, excluding spatial lightmodulator 210 (the LCD) of modulation optical system 200 must provide atotal optical system contrast (Cs) of ˜2,000:1. The actual targetcontrast for the polarization optics does depend on the performance ofthe LCDs. Thus, if for example, the LCDs provide only ˜1500:1 contrast,then the polarization optics must provide ˜3,000:1 contrast. Forexample, an LCD with vertically aligned molecules is preferred for thedigital cinema application due to its high innate contrast. Notably, thecontrast performance of both the LCDs and the polarization opticstypically decrease with increasing numerical aperture of the incidentbeam. Unfortunately, with today's technologies it is not sufficient touse just a single wire grid polarization beamsplitter 240 by itself inorder to meet the 2,000:1 target contrast for the polarization optics.For this reason, modulation optical system 200 also uses a wire gridpre-polarizer 230 and a wire grid polarization analyzer 270 to providethe target polarization performance.

The construction and operation of modulation optical system 200 can beunderstood in yet greater detail, relative to its polarizationperformance. Preferably, pre-polarizer 230 is oriented to transmit “P”polarized light into the modulation optical system. Wire gridpolarization beamsplitter 240 is oriented with its sub-wavelength wirepattern oriented nominally parallel to the sub-wavelength wires ofpolarizer 230 (that is, the two devices are not crossed). Thus, thetransmitted “P” light is further modified (contrast enhanced) bytransmission through the wire grid polarization beamsplitter. Thetransmitted light beam then passes through compensator 260 andencounters spatial light modulator 210, which is nominally a reflectiveLCD, which modifies the polarization state of the incident light on apixel to pixel basis according to the applied control voltages.Intermediate code values, between white and black, reduce the amount of“On” state and increase the amount of “Off” state light. The “On” statelight, which has been polarization rotated, is in the “S” polarizationstate relative to the wire grid beamsplitter 240. Thus the “S” statelight reflects off the wire grid polarization beamsplitter 240, issubsequently transmitted through an optional compensator 265 (see FIGS.5 a and 10) and polarization analyzer 270, and directed to the screen bya projection lens 285. The overall contrast (Cs) for modulation opticalsystem 200 (ignoring the LCD and compensator contributions) can beapproximated by:1/Cs=1/(C _(T1) *C _(T2))+1/(C _(R2) *C _(T3))where C_(T1) is the transmitted contrast of the wire grid pre-polarizer230, C_(T2) and C_(R2) are transmitted and reflected contrast ratios forthe wire grid polarization beamsplitter 240, and C_(T3) is thetransmitted contrast for the wire grid polarization analyzer 270. Inthis system, the overall contrast is largely determined by the lowreflected contrast ratio C_(R2) for “S” polarization state light off ofwire grid polarization beamsplitter 240. The analyzer contrast C_(T3)needs to be quite high to compensate for the low C_(R2) values (˜30:1).Whereas the transmitted contrasts C_(T1) and C_(T2) do not need to beparticularly high, provided that the respective contrast values arereasonably uniform over the spectrum. Polarization analyzer 270 isoriented so that the “On” state light, which reflects off the wire gridpolarization beamsplitter 240 and has “S” polarization relative to thewire grid polarization beamsplitter 240, sees this same light as “P”state light relative to its own structure. Polarization analyzer 270therefore removes any alternate polarization leakage light accompanyingthe desired “On” state beam.

As an example, in green light at 550 nm, wire grid pre-polarizer 230 hasan angle averaged polarization contrast ratio of ˜250:1. When used incombination, wire grid polarization beamsplitter 240 and wire gridpre-polarizer 230 produce an on screen frame sequential optical contrastratio of ˜25:1, which falls way short of the system requirements. Thus,the polarization performance of overall modulation optical system 200 isalso supported with the addition of wire grid polarization analyzer 270,which is nominally assumed to be identical to wire grid polarizer 230.Polarization analyzer 270 removes the leakage of light that is of otherthan the preferred polarization state, boosting the theoretical overallsystem contrast Cs to ˜2900:1. Performance does vary considerably acrossthe visible spectrum, with the same combination of wire grid polarizingdevices providing ˜3400:1 contrast in the red spectrum, but only ˜900:1contrast in the blue. Certainly, this performance variation could bereduced with the use of color band tuned devices, if they wereavailable.

Modulation optical system 200 is best constructed with wire gridpolarization beamsplitter 240 oriented with the surface with thesub-wavelength wires 250 facing towards the spatial light modulator 210,rather than towards the illumination optics (condenser 225) and lightsource (see FIG. 3). While the overall contrast (Cs) is ˜2,900:1 whenthis orientation is used, the net contrast drops precipitously to ˜250:1when the alternate orientation (wires on the surface towards the lightsource) is used. This difference in overall contrast when modulationoptical system 200 is constructed with the image light reflecting offthe wire grid polarization beamsplitter 240, as a function of whetherthe sub-wavelength wires 250 face the spatial light modulator or thelight source, may be less important for slower (larger f#) systems.Additionally, referring to FIG. 3, modulation optical system 200provides the highest contrast and light efficiency when thesub-wavelength wires 250 of wire grid polarization beamsplitter 240 areoriented “vertically” (“into the page”, as shown), rather than“horizontally” (within the plane of the page). Wire grid polarizationbeamsplitter 240 can also be rotated (about the surface normal) by a fewdegrees to tune the contrast performance.

In order to build a digital cinema projector it is necessary tosimultaneously maximize luminance (10,000–15,000 lumens) and contrast(1,000:1+) with a system illuminating 35–55 ft. wide screens, whiledealing with the limitations of the various optics, wire grid devicesand LCDs. Luminance can be maximized by increasing the acceptance angle(numerical aperture) of light incident at the wire grid polarizationbeamsplitter and the LCD. With a wider acceptance angle (or a lower F#),the projection optics are able to gather more light. However, at thesame time, the wider the angle of source light incident at wire gridpolarization beamsplitter, the larger the leakage light from otherpolarization states and thus the smaller the contrast ratio (CR)available. Referring to FIG. 4, there is shown a graph of contrast formodulation optical system 200 (including wire grid pre-polarizer 230,wire grid polarization beamsplitter 240, a VA LCD, and a wire gridpolarization analyzer 270) vs. the F# of the light transmitted throughthe system. The plot of system contrast 300 shows that at approximatelyF/2.3, a contrast ratio of ˜600:1 is achieved. This value issignificantly less than the 1,000:1⁺ contrast needed for digital cinemaprojection. However, efficiency calculations suggest that an LCD baseddigital cinema projector will need to operate below F/3.0 to meet thescreen luminance targets, with systems speeds of F/2.0 to F/2.3 beingpotentially required for the larger screens.

Referring to FIG. 5 a, there is shown a perspective view representinglight polarization states for light reflected by and transmitted throughwire grid polarization beamsplitter 240 within a modulation opticalsystem, for a pixel of LCD 210. A collimated or specular pre-polarizedbeam 350 is transmitted through wire grid polarization beamsplitter 240.As shown in FIG. 5 a, the electric field polarization of transmittedbeam 355 is on a vector perpendicular to the wire grid of wire gridpolarization beamsplitter 240. A returning modulated beam 360 isreflected from the pixel on LCD 210, where the “S” polarized light isthe image data, and the “P” polarized light is to be rejected. Ideally,wire grid polarization beamsplitter 240 transmits 100% of the unwanted“p” light as a modulated transmitted light 370. However, a small leakagelight 365 is reflected from wire grid polarization beamsplitter 240 andaccompanies “s” modulated beam 360, causing reduced contrast (ratio of“s” to “p”). Relative to the modulated beam 360, wire grid beamsplitteracts as a pre-polarizer in transmission and a polarization analyzer inreflection, comprising the typical crossed polarizer configuration.

While some loss of polarization contrast does occur with on axiscollimated light, the effects are more dramatic for oblique and skewrays. To better understand this, FIG. 5 a includes an illustration ofthe beam geometry for a large NA non-specular beam incident on a 45°tilted surface of wire grid polarization beamsplitter 240, while FIG. 5b shows the geometry for a beam incident normal to a surface (such asthe LCD 210, pre-polarizer 230 or analyzer 270). For the normallyincident case, the incoming beam is described by an azimuthal sweep of0–180°, while the polar sweep of angles is limited (0–15° for F/2.0).The oblique rays are those rays that fall in the four quadrants outsidethe axes (azimuthal angles 0° and 180°, 90° and 270°) defined by thecrossed polarizers, and which lie in planes which contain the localoptical axis 275. The skew rays are the rays that lie in planes that donot contain the local optical axis 275. For the case of incidence to the45° tilted surface, the incoming beam is again defined by an azimuthalsweep of 0–180°, while the polar sweep of angles covers ˜0–15° relativeto the optical axis, or a sweep of ˜30–60° relative to the wire gridsurface. This beam geometry will be important in appreciating theresults given by FIG. 8 a–i.

FIG. 6 a illustrates the polarization contrast profile for crossedpolarizers, visible in angular space, and known as the “iron cross”. Theiron cross pattern 320 demonstrates peak extinction in directionsparallel and perpendicular to the grid of the analyzer, and diminishedextinction for the skew rays and oblique rays in the four off-axisquadrants. As the wire grid polarization beamsplitter has superiorangular performance when compared to most existing polarizers, thesedevices have been generally considered to not have a skew ray problem,and therefore to not require further polarization compensation. This isin part because the wire grid polarization beams splitter functions asan O-type polarizer in reflection and an E-type polarizer intransmission, and therefore is partially self compensating when used inboth transmission and reflection as in modulation optical system 200.However, even so, the extinction of the wire grid polarizationbeamsplitter is still not adequate for demanding applications likedigital cinema.

In the original electronic projection systems that were developedutilizing reflective liquid crystal displays, each LCD was addressedfrom behind using a CRT. Today, state of the art reflective LCDs aredirectly electronically addressed by means of a silicon backplane. Thesemodern devices, which are known as liquid crystal on silicon (LCOS)displays, generally comprise a silicon substrate, which is patternedwith pixel addressing circuitry, over coated with reflective and lightblocking layers, followed by an LCD alignment layer, a thin (˜5 μm)layer of liquid crystal, and an anti-reflection (AR) coated cover glass.The inside surface of the cover glass for a VA LCD has an ITO electrodeaddressing layer and an alignment layer on the internal surface,abutting the liquid crystal layer. The optical performance of an LCDdepends on many design parameters, including the material properties ofthe liquid crystals, the electrode structure, the pixel patterning andproximity, the ON state and OFF state orientations of the liquid crystalmolecules, the use and construction of the alignment layers, the opticalproperties of the reflective, anti-reflective, and light blockinglayers, etc. For example, while the liquid crystal molecules arenominally vertical to the inside surfaces of the silicon substrate andthe cover glass, in actuality the surface adjacent molecules areoriented with a residual tilt of 1–2 degrees from the normal. If thisresidual tilt angle becomes larger, device contrast starts to suffer.

The “iron cross” illustration of FIG. 6 a also represents the nominalpolarization response of an ideal VA LCD, as seen through crossedpolarizers, assuming it has a negligible tilt angle. However, the netcontrast provided by the modulation optical system can be degraded byvarious subtle effects within either the LCDs (large tilt angles, biasvoltages for the OFF state, thermally induced stresses, and largeincident angles (large NA's)) or within the polarization devices,including the wire grid polarization beamsplitter (such as wire surfaceorientation, wire rotation, and large incident angles (large NA's).These effects can either cause the contrast to be generally reducedwhile the iron cross pattern 320 is retained, or cause the iron crosspattern 320 to be deformed into another extinction pattern (a “baseball”pattern 325 shown in FIG. 6 b, for example). In the case of modulationoptical system 200, which partially comprises a wire grid pre-polarizer230, a wire grid polarization beamsplitter 240, a vertically aligned LCD210, and a wire grid polarization analyzer 270, the nominal system onlyprovides ˜600:1 contrast in the green at F/2.3, which is belowspecification. The system contrast can be enhanced, to meet and exceedspecification, through the use of the appropriate compensators.Certainly polarization contrast can be potentially enhanced by makingdesign changes to the actual polarization devices (the wire gridpolarization beamsplitter and the LCDs) themselves. However, as it isnot always possible or easy to alter the fundamental design,manufacturing, and performance limitations of these devices, alternatemethods of improving contrast have been sought. In particular, thecontrast performance of modulation optical system 200 has been enhancedwith new polarization compensators developed specifically to work withwire grid polarizers, and with new polarization compensators developedspecifically to work with the combination of vertically aligned LCDs andwire grid devices.

Compensators and polarizers are constructed from birefringent materials,which have multiple indices of refraction. Comparatively, isotropicmedia (such as glass) have a single index of refraction, and uniaxialmedia (such as liquid crystals) have two indices of refraction. Opticalmaterials may have up to three principle indices of refraction. Thematerials with all three different refractive indices are calledbiaxial, and are uniquely specified by its principal indices nx₀, ny₀,nz₀, and three orientational angles as shown in FIG. 7 a. FIG. 7 b showsa biaxial film with the axes of nx₀, ny₀, and nz₀ aligned with x, y, andz axes, respectively. The materials with two equal principal refractiveindices are called uni-axial materials. These two equal indices areordinary index and referred as n₀. The other different refractive indexis called an extraordinary index n_(e). The axis of n_(e) is alsoreferred to as an optical axis. Uniaxial materials are uniquelycharacterized by n_(e), n_(o), and two angles describing the orientationof its optical axis. When all three principal indices are equal, thematerials are called isotropic.

Light sees varying effective indices of refraction depending on thepolarization direction of its electric field when traveling through auniaxial or biaxial material, consequentially, a phase difference isintroduced between two eigen-modes of the electric field. This phasedifference varies with the propagation direction of light, so thetransmission of the light varies with angle when uniaxial or biaxialmaterials are placed between two crossed polarizers. These phasedifferences translate into modifications of the local polarizationorientations for rays traveling along paths other than along or parallelto the optical axis. In particular, a compensator modifies or conditionsthe local polarization orientations for rays at large polar angles,which also includes both oblique and skew rays. A liquid crystalmaterial is typically a uniaxial material. When it is sandwiched betweentwo substrates as in a liquid crystal display, its optic axis generallychanges across the thickness depending on its anchoring at thesubstrates and the voltage applied across the thickness. A compensatoris constructed with one or more uniaxial and/or biaxial films, which aredesigned to introduce angularly dependent phase differences in a way tooffset the angle dependence of phase difference introduced by liquidcrystals or other optics. As is well known in the art, a uniaxial filmwith its optic axis parallel to the plane of the film is called aA-plate as shown in FIG. 7 c, while a uniaxial film with its optic axisperpendicular to the plane of the film is called a C-plate, as shown inFIG. 7 d. A uniaxial material with n_(e) greater than n_(o) is called apositively birefringent. Likewise, a uniaxial material with n_(e)smaller than n_(o) is called negatively birefringent. Both A-plates andC-plates can be positive or negative depending on their n_(e) and n_(o).A more sophisticated multi-layer compensator 400 has its optic axis orthree principal index axes varying across its thickness, as in FIG. 7 e,where a stack of compensation films (birefringent layers 410 a, 410 b,and 410 c) are used with a substrate 420 to assemble the completecompensator. A detailed discussed of stack compensation can be found inU.S. Pat. No. 5,619,352 (Koch et al.). As is well known in art, C-platescan be fabricated by the use of uniaxially compressed polymers orcasting acetate cellulose, while A-plates can be made by stretchedpolymer films such as polyvinyl alcohol or polycarbonate.

The combination of crossed wire grid polarizers (wire grid polarizationbeamsplitter 240, wire grid pre-polarizer 230, and wire gridpolarization analyzer 270) in modulation optical system provides anexcellent dark state for light traveling in the planes parallel orperpendicular to the wires. However, a maximum amount of light leakageoccurs when light travels at a large polar angle (theta) away from thepolarizer normal direction and 45/135 degree relative to the wires (FIG.5 b shows the polar and azimuthal geometry for the polarizers). Forexample, with reference to FIG. 6 a, for the standard “iron cross” typeextinction pattern, peak contrast along the axes can exceed 1,000:1,while contrast in the four quadrants located 45 degrees off the crossedcoordinate axes falls off to 300:1 or less. Light transiting theseangular regions, which includes skew rays, experiences less extinctionthan light closer to the axes. This loss of contrast from the quadrantrays and the skew rays can be significant for digital cinema projection,which again requires high optical system contrast (>2,000:1) and fastoptics (<F/3.0).

Wire grid polarizers have been studied by the use of effective mediumtheory (“Generalized model for wire grid polarizers”, Yeh, SPIE Vol.307, (1981), pp.13–21). When the grating pitch (p) is much smaller thanthe wavelength (λ), the sub-wavelength grating can be approximatelyconsidered as an uni-axial film with effective refractive indices.Although effective medium theory is much easier to be implemented andprovides a qualitative understanding of wire grid polarizers, itgenerally fails to obtain accurate results. It is especially true forcalculation of very low transmission through crossed wire gridpolarizers. The limitation of effective medium theory has been pointedout by Kihuta et al. (“Ability and limitation of effective medium theoryfor sub-wavelength gratings”, Optical Review 2, (1995) pp.92–99). As aresult, the wire grid polarizers have been modeled using the moreexacting rigorous coupled wave analysis (RCWA) discussed in Kuta et al.(“Coupled-wave analysis of lamellar metal transmission gratings for thevisible and the infrared”, Kuta, et al., Journal of the Optical Societyof America A, Vol. 12, (1995), pp.1118–1127). The results given in FIGS.8 a through 8 i for wire grid polarizers are modeled using RCWA.

FIG. 8 a shows the theoretical transmission through crossed wire gridpolarizers about normal incidence, and shows that the transmission at apolar angle of 20 deg. (F/1.5) and an azimuthal angle of 45 deg. is0.99×10⁻³, which is 2.5× larger than the transmission of 0.4×10⁻³ at apolar angle of 0 deg. For an even larger polar angle, such as 40 deg.(F/0.8), at an azimuthal angle of 45 deg., the transmission loss is muchgreater, with the value of 5×10⁻³. The increased transmission translatesinto additional light leakage, and thus loss of contrast. For thesecalculations, the wire grid polarizers were modeled as aluminum wirestructures, deposited on Corning glass 1737F, with a wire pitch of 144nm (˜λ/4), a wire duty cycle of 0.45, and a wire height of 130 nm. Thewire grid polarizer is modeled in the green at 550 nm, with therefractive index of Al being 0.974+i6.73, and the refractive index ofCorning glass is 1.52. These parameters are used for FIG. 8 a throughFIG. 8 i unless specified otherwise. As can be seen in FIG. 8 a, themaximum light leakage (reduced contrast) occurs at 45 degrees relativeto the wire grid. FIG. 8 a can be understood with reference to thegeometry of FIG. 5 b, which shows that for the normally incident beam,the relevant cone of light is described by an azimuthal sweep of 0–180°and a polar sweep of ˜0–20° (F/1.5). The plot of FIG. 8 a showsvariations in transmission for crossed polarizers vs. azimuthal andpolar angles, rather than the variations in contrast. Polarizationcontrast can be difficult to model in a comprehensive way for a complexsystem like modulation optical system 200. However, contrast isapproximately inversely proportional to the transmission for crossedpolarizers, such that small changes in transmitted light can cause hugechanges in system contrast. While FIGS. 8 a–f also show significantoff-angle transmission effects for slower beams (in particular at 10°,or F/2.9), the data will be consistently presented at 20° (F/1.5) for amore dramatic comparison.

Notably, the general behavior of crossed polarizers to suffer lightleakage for oblique and skews rays at large polar angles does not changesubstantially just by using better polarizers. For example, modeling hasshown that even if the pitch of wire grid is much smaller than thewavelength of the light, such as 1/100, a significant amount of lightstill leaks through two crossed wire grid polarizers at large polarangles. FIG. 8 b shows the transmission through crossed wire gridpolarizers, where the pitch of the wire grid polarizers is 5.5 nm(λ/100). Certainly the fine pitch λ/100 device does show lowertransmission than does the λ/4 device (0.23×10⁻³ vs. 0.4×10⁻³ at a polarangle of 0 deg.), and thus provides higher contrast (the theoreticalcontrast differences are much greater than 2× between λ/100 and λ/4devices). In this case, the modeled λ/100 device shows increasedtransmission (and thus light leakage) at a polar angle of 20 deg. and anazimuthal angle of 45 deg. of 0.95×10⁻³, which is ˜4× larger than thetransmission of 0.23×10⁻³ at a polar angle of 0 deg. At a polar angle of40 degrees (and an azimuthal of 45 degrees), the transmission (lightleakage) is 10× greater (9.7×10⁻³). Thus, even for these λ/100 wiregrids, which are far finer than what is presently manufacturable, theoff axis behavior is largely the same, although the theoreticalextinction is greater.

Wire grid polarizers, which transmit the P-polarization as anextraordinary ray (E-type) and reflect the S-polarization as an ordinaryray (O-type), while only absorbing ˜10% of the incident light, cannot beaccurately treated as a uniaxial film. By comparison, the standard sheetpolarizer, which is manufactured by Polaroid Corporation, is similar tothe wire grid polarizer in that it uses “wires” (iodine atoms imbeddedin stretched PVA plastic), is actually a significantly different device.First, the sub-wavelength “wires” (p<<λ) of the dye sheet polarizer aresignificantly smaller than wires of the visible wavelength wire gridpolarizer (p˜/λ4). Moreover, the dye sheet polarizer is an O-typepolarizer, which transmits the ordinary wave and absorbs (rather thanreflects) the extraordinary wave. The standard dye sheet polarizer canbe accurately modeled as a uniaxial film with an extraordinary index andan ordinary index. Optiva Inc. recently developed an E-type sheetpolarizers based on supra-molecular lyotropic liquid crystallinematerial, which transmit the extraordinary wave and absorb the ordinarywave of incident light. (see Lazarev et al., “Low-leakage off-angle inE-polarizers”, Journal of the SID, vol. 9, (2001), pp.101–105). TheOptiva polarizer is a sheet polarizer similar to the standard dye sheetpolarizer, except that it is an E-type polarizer which transmits theextraordinary wave and absorbs (rather than reflects) the ordinary wave.

When two standard O-type dye sheet Polaroid polarizers are used in thecrossed configuration, an iron cross pattern 320 (see FIG. 6 a) is alsoexperienced. Light leaks through these conventional crossed sheetpolarizers at obliquely incident angles with maximum leakage occurringat 45 degrees relative to the transmission or absorption axes of thesheet polarizers. Various compensators have been proposed to reducelight leakage through crossed O-type polarizers, as published by Chen etal. and in Uchida et al. (T. Ishinabe, T. Miyahita, and T. Uchida,“Novel Wide Viewing Angle Polarizer with High Achromaticity”, SID 2000Digest, pgs. 1094–1097). According to Chen, a combination of uniaxialmaterials, an A-plate and a C-plate, dramatically reduces light leakageat off angle. One of its design requirements is that the optical axis ofthe A-plate should be parallel to the transmission axis of the adjacentpolarizer. Uchida solves the same problem using two biaxial films toconstruct the compensator.

Although wire grid polarizers (E-type polarizer in transmission, O-typein reflection) and standard sheet polarizers (O-type in transmission,E-type absorption) are significantly different devices, benefit might beobtained by combining an existing compensator for a sheet polarizer withcrossed wire grid polarizers. FIG. 8 c shows the transmission throughcrossed wire grid polarizers paired with a prior art sheet polarizercompensator from Chen at al., which consists of a 137 nm A-plate and a80 nm C-plate. For the case of a light beam at a polar angle of 20 deg.(F/1.5) and an azimuthal angle of 45 degrees, this compensator doesprovides significant improvement, reducing the transmission to0.52×10⁻³, which represents about 30% more light leakage than theon-axis case for an un-compensated crossed polarizers (0.4×10⁻³).However, at greater polar angles, such as 40 degrees (again, anazimuthal angle of 45 degrees) this compensator still allowssubstantially greater transmission, at a level of 2.4×10⁻³, or ˜6×greater than the on-axis case. The pitch of the wire grid is againassumed to be 144 nm. Thus, while prior art sheet polarizer compensatorscan be used in combination with crossed wire grid polarizers to providesome polarization contrast improvement, there is yet room for furtherimprovement.

Fortunately, it is possible to design compensators which arespecifically optimized to work with wire grid polarizers and wire gridpolarization beamsplitters, and which can be used to boost the contrastprovided by modulation optical system 200. When wire grid polarizers areutilized as a polarizing beamsplitter, they first transmit light andthen reflect light, or first reflect light and then transmit light. Theangle at which light strikes the wire grid polarizer at the first timeis generally different from the angle at which light does at the secondtime. The new compensators have been developed to minimizing lightleakage through crossed wire grid polarizers at off angles withinmodulation optical system 200. Likewise, compensators have beendeveloped which reduce light leakage through a wire grid polarizingbeamsplitter.

As a first example, a polarization compensator was designed as acombination of an A-plate and C-plate, neither of which will affect theon-axis transmission while reducing the off-axis transmission. Thedesigned compensator, which enhances the performance of crossed wiregrid polarizers (wire grid pre-polarizer 230 and wire grid polarizationanalyzer 270 of FIG. 3), uses a combination of two specific birefringentfilms, a +275 nm A-plate and a −60 nm C-plate. FIG. 8 d, which shows thetotal transmission through the combination of the crossed wire gridpolarizers and this first example compensator, shows a broad change inthe transmission response curves, indicating significant transmittedlight reductions as compared to FIG. 8 a. The transmission is below0.48×10⁻³ for all polar angles up to 40 deg. (˜0.4×10⁻³ at 20° polarangle), which is basically equal to the on-axis transmission for theun-compensated crossed wire grid polarizers (0.4×10⁻³). In actuality,the compensator modifies or conditions the polarization orientations ofthe oblique and skew rays to improve their transmission through thecrossed polarizers, thereby enhancing the contrast of the modulatedbeam. This optimized compensator for wire grid polarizers also providessignificantly better performance than does the sheet polarizercompensator discussed previously. Notably, the optical axis of theA-plate for this wire grid polarizer compensator is perpendicular to thetransmission axis of the adjacent polarizer. Whereas, by comparison, theprior art sheet polarizer compensator, as described by Chen et al.,requires that the optical axis of the A-plate to be parallel to thetransmission axis of the adjacent polarizer.

Although, this first example compensator design has significantlyimproved the performance of a modulation optical system 200 which usescrossed wire grid polarizers, where these wire grid devices have arelatively large pitch (p=144 nm ˜λ/4), the same compensator design canimprove the performance when wire grid devices with a smaller pitchesare used. For example, FIG. 8 e shows the modeled performance of a finepitch device (p=5.5 nm ˜λ/100) with compensation, where the transmissionat a polar angle of 40 deg. and an azimuthal angle of 45 deg. hasdropped to 0.24×10⁻³ as compared to the prior un-compensated result of9.7×10⁻³ shown in FIG. 8 b.

A second example compensator was designed for use with crossed wire gridpolarizers, which also has a combination of an A-plate and a C-plate. Inthis case, the A-plate and C-plate both have positive birefringence,with retardations of 137 nm and 160 nm, respectively. Unlike the firstexample compensator, the optical axis of the A-plate for thiscompensator is parallel to the transmission axis of the adjacentpolarizer. FIG. 8 f shows the improved transmission resulting from thiscompensator design, which is below 0.46×10⁻³ for all polar angles up to20 degrees. However, at a polar angle of 40 deg. and an azimuthal angleof 45 deg. the transmission is only reduced to 1.1×10⁻³. While thisdesign is not as good as the first example compensator design,particularly above 20 degree polar angle (see FIG. 8 d), the lightleakage is still significantly reduced as compared to the un-compensatedcrossed polarizers (see FIG. 8 a). As before, this compensator can beinserted into a modified the modulation optical system 200 of FIG. 10,as an added element, secondary compensator 265.

In FIG. 10, which shows modified modulation optical system 200, thecompensator used to optimize performance through the crossed wire gridpolarizers (pre-polarizer 230 and analyzer 270) is located prior toanalyzer 270, and is shown as secondary compensator 265. This samecompensator could alternately be located just after wire gridpre-polarizer 230, as indicated by alternate secondary compensator 266of FIG. 10. As another alternative, part of a designed compensator forthese crossed polarizers can be positioned as secondary compensator 265,while another portion is simultaneously provided as alternate secondarycompensator 266. That is an unlikely scenario, as both the componentcount and mounting requirements are increased. It is also a requirementthat the secondary compensator(s) 265 (and/or 266) be located in theoptical path between wire grid pre-polarizer 230 and wire gridpolarization analyzer 270. That means that secondary compensator 265cannot, for example, be located after wire grid polarization analyzer270.

Secondary compensator 265 can also be used in an unfolded optical systemwithout a polarization beamsplitter, as shown in FIG. 5 c. In this case,transmitted polarized light exits wire grid pre-polarizer 230, passesthrough a spatial light modulator 210 (which is nominally a transmissiveLCD), secondary compensator 265, and wire grid polarization analyzer270. Alternately, the wire grid polarizer secondary compensator 265 canbe located prior to the spatial light modulator 210 within modulationoptical system 200. As shown in FIG. 5 c, the wire grid pre-polarizer230 and wire grid polarization analyzer 270 are crossed, so thatmodulation optical system 200 is nominally in the Off state, and spatiallight modulator 210 rotates light to transmit through the wire gridpolarization analyzer 270 to provide On state light. It should beunderstood that the wire grid pre-polarizer 230 and wire gridpolarization analyzer 270 can be aligned for nominal open statetransmission (not crossed), with the spatial light modulator 210rotating light for the Off state.

Polarization response improvement can also be provided for the wire gridpolarization beamsplitters, as well as for the wire grid polarizers.FIG. 8 g shows the combined transmission (product of the transmittedlight and reflected light) through wire grid polarizing beamsplitterswithout polarization compensation, assuming that the spatial lightmodulator 210 is replaced with a perfect mirror. In this case, theincoming beam is incident on 45° tilted surface with a cone described byan azimuthal sweep of 0–180°, and a polar sweep of angles of ˜0–40° (seeFIG. 5 a), where the light falls within 0–15° polar angle for an F/2.0beam. For example, FIG. 8 g. shows a combined transmission withoutpolarization compensation of 6.5×10⁻² at a polar angle of 30° and anazimuthal angle of 45°.

A third example compensator was designed, in this case to enhance thecontrast provided by wire grid polarization beamsplitter 240, as used inthe modulation optical system 200 of FIG. 10 along with spatial lightmodulator 210 (VA LCD). This compensator example has a combination of anA-plate and a C-plate, having retardations of 90 nm and 320 nm (bothwith positive birefringence), respectively. Within the layered structureof the compensator, the A-plate is preferentially located closer to thewire grid polarization beamsplitter than the C-plate, which is closer tothe LCD. The optical axis of A-plate is parallel to the transmissionaxis of the adjacent polarizer (perpendicular to the wires). FIG. 8 hshows the combined transmission through a wire grid polarizingbeamsplitter used in combination with this compensator is reduced to2.7×10⁻² compared to 6.5×10⁻² at a polar angle of 30 degrees in FIG. 8g. Even at smaller polar angles, such as 15 or 20 degrees, thecompensator reduces transmission (less leakage) by ˜2× as compared tothe un-compensated wire grid polarization beamsplitter. This compensatoris shown in the modified modulation optical system 200 of FIG. 10 ascompensator 260, and is located between wire grid polarizationbeamsplitter 240 and liquid crystal spatial light modulator 210. This isthe only acceptable location for this compensator within modulationoptical system 200.

A fourth example compensator was designed, as with the last exemplarydevice, to enhance the combined transmission provided by wire gridpolarization beamsplitter 240 used in the modulation optical system 200of FIG. 10 along with spatial light modulator 210 (VA LCD). Thiscompensator is a combination of A-plate and C-plate having a retardationof 90 nm and −200 nm, respectively (positive and negativebirefringence). The compensator of FIG. 8 i provides a smaller combinedtransmission, which is 3.5×10⁻² compared to 6.5×10⁻² in FIG. 8 g. Unlikethe third example compensator, the optical axis of the A-plate for thiscompensator is perpendicular to the transmission axis of the adjacentpolarizer (parallel to the wires), rather than parallel to thetransmission axis (perpendicular to the wires). As before, thiscompensator is shown in the modified modulation optical system 200 ofFIG. 10 as compensator 260.

It should be emphasized that the prior art does not describe how todesign a modulation optical system for a projection display using bothLCDs and wire grid devices, which further has polarization compensatorsto boost contrast. Certainly, the actual exemplary compensators designedfor use with the wire grid devices can have conventional structures andcombinations of materials (such as polycarbonate or acetate) as havebeen previously described for other polarization devices. However, wiregrid polarizers are distinctly different from the prior art devices(sheet polarizers and MacNeille prisms for example) in subtle andnon-obvious ways, and therefore the design of the associated optimizedcompensators cannot be easily extrapolated from the prior compensatordesigns.

It is of course understood that various designs can achieve comparableperformances as described above or even better. It is also understoodthat a single biaxial film can be used to replace the combination ofA-plate and C-plate for any of these exemplary compensators. It shouldalso be understood that the modeled compensators can be designed inreverse order, with the C-plate encountered before the A-plate, ratherthan the order of A-plate and then C-plate provided in the aboveexamples. When the order is switched, the designed birefringence valueslikely change. It is also understood that additional A-plate and/orC-plate and/or biaxial films can be added to the combination of A-plateand C-plate for any of these exemplary compensators.

Certainly, as with the addition of any other optical component into asystem, the usual concerns for providing the mounting and AR coatingsfor these compensators also apply. The compensators may be constructedwith their birefringent films sandwiched between two glass substrates,with optical matching adhesives or gels holding the elements together.In that case, any glass to air surfaces should be AR coated.Alternately, the compensators can be integrated with the wire gridpolarizers (wire grid pre-polarizer 230 and wire grid polarizationanalyzer 270) and mounted directly to the glass substrates of thesecomponents. That reduces the part count, the count of glass to airsurface interactions, and the mounting issues. However, the compensatorshould be mounted to the flat glass surface of the wire grid device, andnot to the surface with the wire grid coating.

Although the above examples are designed for a single wavelength at 550nm, it should be understood that these examples function for all otherwavelengths equally well as for 550 nm provided that the material of thecompensator has a dispersion matched with wavelength. This means thatratio of the retardation/wavelength is substantially unchanged acrossall visible wavelengths.

It should also be understood that modulation optical system 200 can beconstructed in a variety of combinations. As depicted in FIG. 10, itincludes wire grid pre-polarizer 230, wire grid polarizationbeamsplitter 240, wire grid polarization analyzer 270, compensator 260,secondary compensator 265, and alternate secondary compensator 266.However the system could be constructed with wire grid pre-polarizer230, wire grid polarization beamsplitter 240, wire grid polarizationanalyzer 270, and compensator 260, with the secondary compensators leftout. Likewise, the system could be constructed with wire gridpolarization beamsplitter 240 and compensator 260, but with wire gridpre-polarizer 230 and wire grid polarization analyzer 270 as non-wiregrid devices, and with the secondary compensators left out. Alternatelyagain, the system could be constructed with wire grid pre-polarizer 230,wire grid polarization beamsplitter 240, wire grid polarization analyzer270, and secondary compensator 265, but with compensator 260 left out.Needless to say, yet other combinations of components are possible.

The overall contrast performance of modulation optical system 200 ofFIG. 10 can be enhanced not only be providing compensators whichoptimize the performance of the crossed wire grid polarizers or the wiregrid polarization beamsplitter, but also which enhance the performanceof the LCDs as seen through the wire grid polarization beamsplitter. Bycomparison, in the prior art, U.S. Pat. No. 5,576,854 (Schmidt et al.) acompensator is described which optimizes for the VA LCD working incombination with a MacNeille beamsplitter. As disclosed in U.S. Pat. No.5,576,854, a 0.27 λ compensator is used, where 0.25 λ's compensate forthe MacNeille prism and 0.02 λ's for birefringence in the counterelectrode substrate. The counter electrode substrate is susceptible tothermal gradients that cause stresses within the substrate, which inturn cause localized birefringence. Even with carefully chosen materialsfor the substrate glass, such as SF-57 or fused silica, a smallretardance, such as 0.02 λ's was used to compensate for residual lightleakages in the dark state from stress birefringence. When a verticalaligned LCD is in a non-active state without any voltage applied, thelight leakage at on axis is small. However, in practice the dark stateis a state with a non-zero voltage called as Voff. This voltage causesthe liquid crystal to tilt down, and can significantly increases lightleakage. Compensators have also been designed by others for example,U.S. Pat. No. 5,298,199 (Hirose, et al.) to correct for this effect.

In the case of a vertically aligned LCD combined with a wire gridpolarizing beamsplitter, the 0.25 λ's retardance used in U.S. Pat. No.5,576,854 for the MacNeille type prism is not required. However, theresidual 0.02 λ's retardance (˜11 nm), which is provided as an A-plate,may still be useful to correct to stress birefringence within the VALCD, even with wire grid devices. In addition, a compensator optimizedfor a VA LCD may also include a negative C-plate when used in fastoptical systems, including a digital cinema system operating at F/3.0 orbelow. Thus, preferred compensators for reflective VA LCD's used incombination with wire grid polarizers comprise a negative C-plate and apositive A-plate. The negative C-plate is preferred to have same amountof retardation as the liquid crystal (+233 nm for example), but withopposite sign, to correct the viewing angle dependence of the liquidcrystal. This viewing angle dependent retardation present in the liquidcrystal is typically ˜160–250 nm.

As an example, FIG. 9 a shows the contrast contour plot for lightreflected off of an ideal VA LCD through crossed polarizers in the Offstate. This corresponds to the “iron cross” pattern 320 of FIG. 6 a,with minimal light along the optical axis (center of the sphericalpattern) and along the directions parallel or perpendicular to thetransmission axis of the crossed polarizers. However, as the iron crosspattern 320 shows, some leakage light can be expected in the fourquadrants. FIG. 9 b shows contrast contour plot for light reflected offa VA LCD with 10 nm residual retardation from induced birefringence inthe substrate, which corresponds to the baseball pattern 325 of FIG. 6b. Unfortunately, when this baseball pattern 325 occurs, leakage lightinto the projection system is significantly increased, and the contrastis reduced. FIG. 9 c shows contrast contour plot for a VA LCD withproper compensators (−233 nm C-plate) designed according to the presentinvention inserted at the above discussed locations. The 1000:1iso-contrast curve extends to more than 13° of polar angle. Thiscompensator can be inserted into modified optical modulation system 200of FIG. 10, immediately prior to the LCD 210, as compensator 260.

In actuality, the compensators for the wire grid polarizationbeamsplitter 240 and the LCD 210 are co-located between these twocomponents, and can be combined into one packaged compensator device.The exemplary compensator for the wire grid polarization beamsplitter240 corresponding to FIG. 8 h used a combination of an A-plate and aC-plate having a retardation of 90 nm and 320 nm. By comparison, thevertically aligned LCD has a retardation of ˜233 nm, and requires aC-plate with a −233 nm retardation for correction. When these twoC-plate designs are combined, the remaining C-plate has only ˜87 nmretardance. Thus, the cancellation between the compensator and the LCDsignificantly decreases the amount of additional retardance needed. Thecombined compensator 260 then comprises the 11 nm A-plate for the VA LCD(0.02 λ's compensation), the 87 nm C-plate, and the 90 nm A-plate forthe wire grid polarization beamsplitter 240 in sequential order, withthe 11 nm A-plate located closest to the LCD 210. The two A-platescannot be simply combined, as the 11 nm A-plate needs to be rotatable,while the 90 nm A-plate has a fixed orientation relative to thesub-wavelength wires 250. Thus, what is provided is an apparatus andmethod for achieving high levels of contrast by using a wire gridpolarization beamsplitter with a compensator for minimizing leakagelight in the pixel black (OFF) state for a VA LCD.

FIG. 4 shows a graph 310 of the compensated contrast that relates systemcontrast to the relative F# for a modulation optical system comprising aVA LCD, wire grid polarizers, a wire grid polarization beamsplitter, anda compensator, which correct for the unwanted P polarization inreturning modulated beam. In this case, a customized version ofcompensator 260 is used. Notably, although use of a compensator canactually reduce CR at higher F# values, the compensator improvescontrast at low values, below approximately F/4.0. Note that compensatedcontrast 310 may not always be better, because compensators can becomplex structures, which can suffer undesired reflections and defects.

It should be understood that the polarization compensation conceptsdeveloped within this application for optimizing the polarizationperformance of wire grid polarizer devices could be used in modulationoptical systems which have spatial light modulators other thanvertically aligned LCDs. For example, spatial light modulator 210 couldalso be a 60 degree twisted nematic LCD, a PLZT modulator, or some otherpolarization rotating modulator.

Thus far, the discussion of the use of polarization compensators in aprojection system using wire grid polarizers generally, and a wire gridpolarization beamsplitter 240 specifically, in combination with a LCDtype spatial light modulator 210, has been directed largely at thedesign and performance of the compensators. Specifically, the goal hasbeen to maximize the frame sequential polarization contrast between themodulated and un-modulated light, by optimizing the variablepolarization response vs. angle (for example, see FIGS. 9 a–c). However,as shown in FIG. 11, which shows a contrast pattern 500 with a highcontrast region 510 in the center. Contrast pattern 500 can be observedon screen in a projected dark state image, where for example, the highcontrast region 510 may exhibit 2,000:1 contrast, while the low contrastregions 520 may only have 800:1 contrast. Contrast pattern 500 isillustrative only, and it should be understood that the low contrastregions may not be experienced symmetrically as shown, nor fall toidentical low contrast values.

Spatial contrast variation at the screen can originate with many sourcesin the system, including LCD variation, compensator retardancevariation, non-telecentric light propagation through the polarizationoptics, and stress birefringence in the polarization sensitive optics.Other non-polarization related factors, such as ghost reflections,surface reflections, and surface scatter, can affect contrast (both ANSIin-frame contrast and frame sequential contrast) in a spatially variantmanner. However, a system with only 150:1 ANSI in-frame contrast canexhibit 2,000:1 frame sequential contrast, as well as a spatiallyvariant frame sequential contrast pattern 500, such as depicted in FIG.11. Assuming that the optical system is telecentric, and also thatstress birefringence in the optics (such as the polarizationbeamsplitter and the recombination prism) is minimized, then any framesequential contrast spatial variation will likely originate in thepolarization compensators and/or in the LCD panels.

The spatial contrast variation represented in FIG. 11 represents aspatial variation in the modulated ON state light combined with aspatial variation of the leakage light, which is light of the orthogonalpolarization state to the modulated ON state light. Fundamentally, thespatial variation in polarization rotations occurs because the lightdoes not encounter uniform retardances across the image field. It isgenerally understood that retardance is the delay of one polarizationrelative to the orthogonal polarization, where the delay translates intoa phase change Δφ in the polarization of the incoming light. The phasechange Δφ can be calculated asΔφ=2π*t*Δn/λ,where (Δn) is the index change (Δn=n_(∥)−n_(⊥)) (intrinsicbirefringence) provided by the structure and (t) is the thickness of thestructure. Retardance is the phase change Δφ expressed as distance; forexample a π/2 phase change Δφ corresponds to a quarter wave λ/4retardance, which at 550 nm equals ˜138 nm retardance.

Bulk crystaline materials, such as calcite, have an intrinsicbirefringence, as defined above. Liquid crystalline materials also areintrinsically birefringent on a molecular level. However, the actualretardance experienced is dependent on the liquid crystal composition,the orientation (tilt and twist) of the LC molecules within a cell, andthe applied voltage across the cell, as well as the polarization stateof the incident light. Thus, the in-plane birefringence and retardancecan be spatially variant through the thickness of the cell, as well asacross the length and width of the cell. In the case that a display seesnominally uniform drive voltages, the birefringence and retardance canbe considered to be uniform across the length and width of the cell (atleast locally), and the variation through the thickness of the cell willremain. The cell can then be approximated by an effective birefringence(Δn_(eff)=n_(x)−n_(y)) and an effective retardanceΔR=t*Δn _(eff).The retardance of the LC material is specified by both the magnitude ofthe refractive indices (nx, ny) and their relative orientation (the slowaxis of the material corresponds to the larger index). Althoughpolarization compensators can be constructed with bulk intrinsicbirefringent materials, or with form birefringent structures, they canalso be made with liquid crystal polymer type materials that exhibitlocal variations in both refractive index and axial orientation. In thecase that liquid crystal polymers are used to fabricate the compensator,the effective retardance depends on both the magnitude of the fast andslow axes refractive indices, as well as their relative orientation.

As discussed previously, compensators can comprise a series of stretchedpolymer films assembled into a stack assembled onto a substrate (seeFIG. 7 e) with intervening layers of low stress adhesive to hold thestructure together. In the particular instance of a compensatorconstructed with stretched polymer sheets, obtaining small amounts ofin-plane (XY) retardance can be difficult, because the mechanicalstretching process is uneven. For example, a multi-layer compensatorthat was constructed to provide a nominal 10.5 nm in-plane retardanceactually was spatially variant, with retardances ranging from ˜9.0 to12.0 nm (+/−15%). However, polarization compensators can be fabricatedby a variety of means, with potentially less variation. As an example,it has been reported that liquid crystal polymer compensation layers canbe spun coat onto an alignment layer (formed on a substrate), such thatthe retardation values can be controlled within 3% accuracy.Compensators can also be constructed using optical coating,photo-lithographic, and laser etching fabrication technologies to formrobust inorganic structures with much finer control of retardanceuniformity. For example, compensators can be made as an optical thinfilm coating (see for example, U.S. Pat. No. 5,196,953 (P. Yeh)) or asan anisotropic form birefringent optical structure.

The spatial retardance variations present in LCDs can be at leastcomparable (+/−15%) to the largest changes cited above for thecompensators. However, as LCDs are much more complicated structures, itis much more difficult to control or minimize the spatial variation ofretardance within a LCD panel than it is in a compensator. Retardancevariation in LCD's can be caused by a multitude of sources, includingcell gap thickness variation, variations in the LC pre-tilt angle, indexand thickness variations in the dielectric stacks of the internalanti-reflection coatings, and voltage noise from variations in theunderlying CMOS circuitry. Accordingly, improved performance could berealized in a projector by providing a polarization compensator with apatterned retardance that corrects for the retardance variation of theassociated spatial light modulator (LCD).

Certainly patterned retarders or compensators have been described in theprior art. U.S. Pat. No. 5,548,427 (May) shows a spatially variantpatterned retarder, comprising regions of alternately orientedretardances. Other examples include U.S. Pat. No. 5,499,126 (Abileah etal.), which provides color tuned retarders that are aligned with RGB LCDsub-pixels, U.S. Pat. No. 6,496,287 (Seiberle et al.) which provides apattern of spatially variant retarders for counterfeit protection, andU.S. Pat. No. 6,496,239 (Seiberle) which provides a patterned retardermask useful for making other patterned retarders. As another example,the paper “Novel High Performance Transreflective LCD with a PatternedRetarder” by S. J. Roosendaal et al., and published in the SID 03 Digest(pgs. 78–81) describes a patterned retarder that provides differentretardances for the transmissive and reflective portions of atransreflective LCD pixel. The fabrication of the type of patternedretarders described in Roosendaal et al. and in Seiberle '239 aredescribed in some detail in two related papers; “Technologies TowardsPatterned Optical Foils” by B. van der Zande et al. (published in theSID 03 Digest, pgs. 194–197) and in “Photo-Aligned Anisotropic OpticalThin Films” by H. Seiberle et al. (published in the SID 03 Digest, pgs.1162–1165). Both of these references describe the use of polarized UVlight to expose a photo-alignment layer, which is in turn coated with aliquid crystal polymer, to form the patterned retarders. Notably, thisprior art does not describe the use or fabrication of a patternedretarder that is fabricated to have a spatially variant retardance thatcompensates for the spatially variant retardance of a spatial lightmodulator (LCD). Moreover, the prior art does not describe the use of aspatially variant retarder/compensator that is used in combination withan LCD, and in further combination with wire grid polarizers, such thatuniform contrast (gray levels and black) can result with each pixelnominally driven to identical code values. Finally, the prior art doesnot describe the fabrication of such a patterned compensator with otherthan UV exposing light.

An exemplary exposure system 530 is shown in FIG. 12, which is providedto support the fabrication of a patterned compensator 550. Accordingly,spatial light modulator (LCD) 210 is illuminated with an illuminationlight beam, which can be polarized by pre-polarizer 230 and a wire gridpolarization beamsplitter 240 (nominally both wire grid polarizers).Polarized light is then incident on LCD 210, which is electricallydriven on a per pixel basis to identical code values for a nominallyuniform output (nominally a mid-level gray). The modulated lightreflected off the LCD 210 is separated from the greater portion of theunmodulated light by the wire grid polarization beamsplitter 240. Themodulated image bearing light 290 is then directed to the wire gridpolarization analyzer 270, which can also be a wire grid polarizer,where further off state leakage light is removed. Wire grid polarizationanalyzer 270 can be an optional component. Imaging relay lens 540, whichshould be nominally telecentric in both object and image space, providesa real image of the LCD 210 to a plane where the patterned compensator550 is located. Assuming the patterned compensator 550 is to be placedin close proximity to the LCD 210, then imaging relay lens 540 operatesat 1× magnification.

Assuming the patterned compensator 550 is to be fabricated with liquidcrystal polymer materials, in a fashion similar to that described in theSeiberle et al. paper, then the nominal process begins with locating acompensator substrate the exposure system 530. This substrate (notshown) is pre-coated with the light sensitive (polymer) photo-alignmentlayer. An image is then projected onto the patterned compensator 550,having a spatially variant contrast pattern 500, of the sort depictedgenerally in FIG. 11, so as to impress the compensator with a patterncorresponding to the spatially variant retardance of the modulator. As afunction of position, the incident light varies both in intensity andpolarization orientation. In accordance with the type of polymer usedfor the photo-alignment, the alignment layer orientation can bedetermined on localized basis by either director patterning or in-situpatterned photo-polymerization (patterning birefringence (Δn) orthickness), where the polarization orientation of the incident lightdetermines the photo-alignment.

The illumination light beam 220 directed at LCD 210 is preferablyvisible light, rather than UV light. To begin with, most types of LCDsare susceptible to damage when exposed to UV light. Additionally, to mapthe visible light response of the LCD 210 to the patterned compensator550, visible illumination light should be used. Moreover, better resultscan be obtained if the illumination light beam is filtered to a colorspectra (red for example) similar to the color spectra that will bedirected onto a given LCD 210 when that LCD is mounted into the workingsystem. To enable this, the polymer materials used to form the alignmentlayer can be enhanced with visible wavelength sensitive polymerphoto-initiators. Note that the exposure system can also be providedwith its own polarization compensator 260, optimally located between thewire grid polarization beamsplitter 240 and the test cell (LCD 210). Forexample, this compensator may provide C-plate compensation to correctfor the angular response of the LCD and/or the polarizationbeamsplitter. This compensator may also provide in-plane (A-plate)compensation, but the provided in-plane retardance is preferably veryuniform, so that the retardance variations of the LCD 210 are notmasked. Once the photo-alignment is completed, the patterned compensator550 is nominally removed from the exposure system 530, and spun coatwith liquid crystal polymers to form the actual retardation layer havingthe desired spatially variant retardance. Typically thereafter, theretardance imparted into a liquid crystal polymer layer is fixed by UVexposure.

In accordance with the present invention, it is intended that patternedcompensator 550 have a spatially variant retardance that correlates withthe spatially variant retardance of the LCD 210, such that a level oruniform retardance results when the two are used in combination. In theprior discussion, the fabrication of a patterned compensator 550 thatutilizes liquid crystal polymers was discussed in some detail. In thatcase the effective retardance at any given position is a function ofboth the magnitude of the refractive indices (nx, ny) and theirorientation (fast axis and slow axis). Patterned compensator 550 canhave a spatially variant effective retardance, wherein the localizedeffective retardances are negative. The intent then is that the negativeretardance is at its greatest magnitude where the LCD 210 has itgreatest positive retardance, and the negative retardance has minimummagnitude where the LCD 210 has its minimum retardance, such that incombination, a uniform retardance is provided. Patterned compensator 550can also have a spatially variant retardance comprising positiveretardances, where there is less retardance where the LCD 210 has moreretardance, and visa-versa, such that a uniform retardance is provided.For best results, patterned compensator 550 is nominally matched withthe specific LCD 210 that was used in its fabrication in the exposuresystem. Moreover, for patterned compensator 550 to be effective, thespatially variant retardance of the associated LCD should be minimallytime variant. Although FIG. 12 shows and exposure system 530 constructedto facilitate the correction of a reflective spatial light modulator(LCD) 210, it should be understood that this system can be altered tooperate with a transmissive polarization modulator as well. In thatcase, the patterned compensator 550 can be placed immediately after theLCD 210 during this patterning fabrication step for the compensator,thereby eliminating the need for imaging relay lens 540 as part of theexposure system 530.

It should then be understood that patterned compensator 550 isultimately placed into a modulation optical system 200 (such as in FIG.3), nominally in close proximity to the matching LCD 210. As patternedcompensator 550 will typically provide correction for slowly varyingspatial changes in retardance, the registration of compensator 550 tothe LCD 210 should not be super critical. Patterned compensator 550 canbe incorporated into a compensator 260, where it may be supplemented byother A-plate and C-plate type compensators as necessary. The patternedcompensator 550 is described herein as a device nominally made withliquid crystal polymers. Experimental data has been published in thepreviously cited paper by Seiberle et al. that shows that thebirefringence of a liquid crystal polymer type polarization compensatoris stable under high light exposure (3 W/cm² for over at least 9,000hours). Nonetheless, for robust operation of polarization compensators,including the patterned compensator of the present invention, inorganiccompensators (such as dichroic and form birefringent devices) would befavored due to their insensitivity to high thermal loads, stressfulambient environments, and UV light exposure.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the scope of theinvention as described above, and as noted in the appended claims, by aperson of ordinary skill in the art without departing from the scope ofthe invention.

PARTS LIST

-   10. Digital projection apparatus-   15. Light source-   20. Illumination optics-   40. Modulation optical system-   45. Pre-polarizer-   50. Wire grid polarization beamsplitter-   55. Spatial light modulator-   60. Polarization analyzer-   70. Projection optics-   75. Display surface-   100. Wire grid polarizer-   110. Conductive electrodes or wires-   120. Dielectric substrate-   130. Beam of light-   132. Light Source-   140. Reflected light beam-   150. Transmitted light beam-   200. Modulation optical system-   210. Spatial light modulator (LCD)-   220. Illumination light beam-   225. Condensor-   230. Wire grid pre-polarizer-   240. Wire grid polarization beamsplitter-   245. Dielectric substrate-   250. Sub-wavelength wires-   260. Compensator-   265. Secondary compensator-   266. Alternate secondary compensator-   270. Wire grid polarization analyzer-   275. Optical axis-   285. Projection lens-   280. Recombination prism-   290. Modulated image-bearing light beam-   300. System contrast-   310. Graph-   320. Iron Cross pattern-   325. Baseball pattern-   350. Pre-polarized beam-   355. Transmitted beam-   360. Modulated beam-   365. Leakage light-   370. Transmitted light-   400. Multi-layer compensator-   410 a. Birefringent layers-   410 b. Birefringent layers-   410 c. Birefringent layers-   420. Substrate-   500. Contrast pattern-   510. High contrast region-   520. Low contrast region-   530. Exposure system-   540. Imaging relay lens-   550. Patterned compensator

1. A spatially patterned polarization compensator, comprising an opticalstructure fabricated with a spatially variant retardance thatcorresponds to the spatially variant retardance of a spatial lightmodulator, such that when said patterned polarization compensator andsaid spatial light modulator are used in combination, within a lightbeam, net retardance of the combination is nominally spatially uniformon a pixel to pixel basis.
 2. A spatially patterned polarizationcompensator according to claim 1 which is fabricated with liquid crystalpolymer materials.
 3. A spatially patterned polarization compensatoraccording to claim 1 which is fabricated with inorganic materials.
 4. Aspatially patterned polarization compensator, fabricated with theassistance of a pre-existing spatial light modulator having a spatiallyvariant retardance, such that said spatially patterned polarizationcompensator has a spatially variant retardance that corresponds to thespatially variant retardance of said pre-existing spatial lightmodulator, with the result that when said patterned polarizationcompensator and said pre-existing spatial light modulator are used incombination within a light beam, net retardance of the combination isnominally spatially uniform on a pixel to pixel basis.
 5. A spatiallypatterned polarization compensator, comprising an optical structurefabricated with a spatially variant retardance that corresponds to thespatially variant retardance of a pre-existing spatial light modulator,such that when said patterned polarization compensator and saidpre-existing spatial light modulator are used in combination within alight beam of a given color band, a nominally spatially uniform netretardance results.
 6. A spatially patterned polarization compensatoraccording to claim 5 which is fabricated with liquid crystal polymermaterials.
 7. A spatially patterned polarization compensator accordingto claim 5 which is fabricated with inorganic materials.
 8. A spatiallypatterned polarization compensator, comprising an optical structurefabricated with a spatially variant retardance that corresponds to thespatially variant retardance of a spatial light modulator, such thatwhen said patterned polarization compensator and said spatial lightmodulator are used in combination within a light beam, net retardance ofthe combination is nominally spatially uniform across the full width ofsaid spatial light modulator on a pixel to pixel basis.
 9. A spatiallypatterned polarization compensator, comprising an optical structurefabricated with a spatially yariant retardance that corresponds to thespatially variant retardance of a spatial light modulator, such thatwhen said patterned polarization compensator and said spatial lightmodulator are used in combination within a light beam of a given colorband, a nominally spatially uniform net retardance results.
 10. Aspatially patterned polarization compensator according to claim 9 whichis fabricated with liquid crystal polymer materials.
 11. A spatiallypatterned polarization compensator according to claim 9 which isfabricated with inorganic materials.